In recent years, it is discussed in the field of lithography technology that exposure light in the wavelength region called the extreme ultraviolet (EUV) region in which the intermediate value is 13.5 nm should be used to meet the demand for the miniaturization of semiconductor devices. Masks for use in EUV lithography are reflection-type masks. The reflection-type mask is produced in the following method. Layers of two types, each having a different reflectance, are alternately laid on a glass substrate, thus forming a multi-layer film. On the multi-layer film, a film called an absorber is formed. The absorber is then processed, whereby a circuit pattern is formed.
As defects inherent to the EUV mask, multi-layer film defects are mentioned. The multi-layer film defects have resulted from the pits or the bumps made on the surface of the substrate. Alternatively, they have resulted from foreign matter that has stuck to the substrate before or during the forming of the multi-layer film. If such foreign matter exists, the cycle of the multi-layer film is disturbed, inevitably resulting in a phase difference with respect to any normal part. This phase difference is known as a phase defect. The multi-layer film must therefore be inspected for phase defects, before the absorber is formed on the multi-layer film called a blank.
If the phase defects are left undetected at the blank inspection, the phase defects of the resultant mask will be exposed from the absorber pattern. A technique of avoiding this is available, which hides the phase defects under the absorber pattern. Even if this technique is applied and if a mask is made from a blank having phase defects known in position and size, the phase detects may be exposed from the absorber pattern when the blank is processed into a mask. In this case, the phase defects must be corrected. However, it is difficult to locate the phase positions in the image the correction apparatus displayed, because the steps resulting from the phase defects are very low, or several nanometers or less, at the surface of the multi-layer film.